Device for ellipsometric two-dimensional display of a sample, display method and ellipsometric measurement method with spatial resolution

ABSTRACT

This invention concerns a device for ellipsometric two-dimensional display of a sample placed in an incident medium, observed between a convergent light cross-reflected analyser and polarizer, wherein the ellipsometric parameters of the ensemble formed by the sample and a substrate whereon it is placed, are processed. The substrate comprises a base and a stack of layers of base and its ellipsometric properties are known. The ellipsometric properties of the substrate are such that the variations of the ellipsometric parameters of the sample are displayed with a contrast greater than the contrast produced in the absence of such substrate. The invention also concerns a display method and an ellipsometric measurement method with spatial resolution.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional of 10/450,958 filed Dec. 23, 2003, nowU.S. Pat. No. 7,209,232, which is a 371 of PCT/FR01?04046 filed on Dec.18, 2001.

FIELD OF THE INVENTION

This invention concerns a device for ellipsometric two-dimensionaldisplay of a sample, a display method and an ellipsometric measurementmethod with spatial resolution. It is particularly well suited todisplay with ellipsometric contrast or interferential contrast.

BACKGROUND OF THE INVENTION

A sample receiving light and reflecting the same will generally modifythe polarisation thereof.

It is possible to use this property to visualise a sample or tocharacterise said sample by measuring its ellipsometric parametersdesignated generally as ψ and Δ.

In this view, one may, for instance, refer to the book by Azzam andBashara published in 1979.

Initially, it has been sought to process the extinction of the Fresnelcoefficient r_(p) at the Brewster angle in order to provide an accurateellipsometric measurement of the parameters ψ and Δ (ellipsometry) orsensitive display of very thin films, notably at the surface of water(Brewster angle microscopy).

Besides, it has been sought to irradiate a zone of a sample under asingle incidence and a single azimuth in order to measure the parametersψ and Δ corresponding to this zone.

The aim within the framework of this invention is to providesimultaneous processing of the parameters ψ and Δ for a number of pointsof a sample, each defined by their coordinates x and y. This is calledellipsometric two-dimensional display or measurement of a sample.

Moreover, this invention relates to small samples that may be observed,displayed or measured under an optical reflection microscope. It may beconventional microscopy, microscopy with differential interferentialcontrast or fluorescence microscopy.

This type of microscopic observations poses particular constraintsinasmuch as, on the one hand, the microscope lenses have a wide digitalaperture which creates observation conditions significantly differentfrom the usual conditions of the ellipsometric measurements wherein thebeams, illuminating beams as well as measuring beams (or reflectedbeams) are generally small aperture collimated beams and, on the otherhand, where the illuminating beams are most often distributed uniformlyaround the normal incidence, i.e. within a range of angles of incidencehardly lending themselves to ellipsometry.

Still, display methods based on the use of an antiglare substrate havebeen suggested previously, but they resort to the “incoherentreflectivity” of the substrate. The substrates suggested previously aretherefore antiglare for a non-polarised light or for a polarised lightwith a constant polarisation direction relative to the plane ofincidence, which is incompatible with the use of a microscope. Theprinciple is based on the minimisation of the second member of theequation (E4).

$\begin{matrix}{{\Phi_{N}\left( {\theta,{NP}} \right)} = {\frac{1}{2}\left( {{r_{p}}^{2} + {r_{s}}^{2}} \right)}} & {E4}\end{matrix}$where r_(p) and r_(s) are the complex reflection coefficients of eachpolarisation on the substrate affected which depend implicitly on x andon y, Φ_(N)(θ,NP) being the normalised flux reflected for an angle ofincidence θ, in non-polarised light.

It is obvious that complete extinction is only possible for|r_(p)=|r_(s)|=0, which is an extremely restricting condition, since thevalues of both Fresnel coefficients are set. The condition of completeextinction, |r_(p)+r_(s)|=0, is far more flexible since it is solelytranslated into a relation between both Fresnel coefficients,r_(p)=−r_(s)  E6

Antiglare substrates for a polarised light have also been suggested inorder to enhance the performances of the ellipsometers, but theellipsometry and the optical microscopy had been considered until now asincompatible.

The aim of the invention is therefore to provide an ellipsometrictwo-dimensional display of an object with very small thickness,invisible under optical microscope under observation conditions known ascompatible with the use of a commercially available optical microscope.

In spite of this, according to the invention, it is possiblesimultaneously to visualise the object and to measure its thickness andits index under microscope.

SUMMARY OF THE INVENTION

To this end, the studied object is deposited on a particular substrate,the association of the studied object and of the substrate forming theset under scrutiny that we call—the sample—. The substrate is designedso that the studied object, albeit very thin, suffices by its presenceto modify the aspect of the substrate, thereby leading to the display ofthe object.

In this view, the substrate consists of a base covered with a stack oflayers so that, on the one hand, the thickness e of the last layerverifies the condition d²/de²[Ln|r_(p)+r_(s)|]=0 and so that, on theother hand, the minimum of the quantity |r_(p)+r_(s)| on the set ofvalues of e is as small as possible.

Similarly, the presence of the object suffices under these conditions tomodify, in a way measurable under optical microscope, the parameters ψand Δ of the substrate, so that the optical characteristics of theobject may be extracted from the measured parameters ψ and Δ of thesample.

Thus, the substrate is designed so that the sensitivity of theparameters ψ and Δ of the sample for a small disturbance of itsconstitutive parameters is very large for small incidence angles,therefore very different from the Brewster angle, while the display andmeasuring methods suggested are designed, moreover, so that the radialgeometry of the microscope has become compatible with the processing ofthese ellipsometric features.

In a preferred embodiment, implementing a differential interferentialmicroscope (DIC) (thanks to a device inserted in the vicinity of therear focal plane of the lens, for example a Nomarski device or a Smithdevice), the illuminating beam, linearly polarised according to theazimuth φ=0, is splitted by the DIC device into two linearly polarisedbeam according to the directions φ=45° and φ=−45° and offset laterally,relative to one another by a small quantity Δd, both wave planesassociated with both these polarisations undergoing upon reflection onthe sample, phase variations due to the presence or the lack ofhomogeneity of the object, whereas these phase variations are convertedinto colour or intensity variations after going, upon the return of thereflected beam, into the DIC device, then in the analysercross-connected with the polarizer. In this mode of observation, thecontrast of the object is optimised thanks to the adjustment of acompensator included in the DIC device. This adjustment consists inswitching off the interference between both beams reflected by thenon-interesting regions of the sample while adjusting their phase-shiftat the device where they produce interference, i.e. at the analyser,whereas the quality of this extinction conditions the quality of thedisplay. The mathematic condition of this extinction is the same aspreviously, i.e. r_(p)+r_(s)=0. The condition of maximum sensitivity onthe thickness e of the last layer of the stack in this mode ofobservation is d²/de²[Ln|r_(p)+r_(s)|]=0.

The display method suggested is therefore globally optimal for all theobservations under microscope between cross-connected polarizer andanalyser, even when a DIC device is included in the microscope.

The invention relates therefore to a device for ellipsometrictwo-dimensional display of a sample, comprising an object, placed in anincident medium, observed between a convergent light cross-reflectedanalyser and polarizer, wherein the ellipsometric parameters of the setcomposed of the object and a substrate whereon it is placed, areprocessed.

According to the invention:

-   -   the substrate comprises a base and a stack of layers and its        ellipsometric properties are known,    -   the ellipsometric properties of the substrate being such that        the variations of the ellipsometric parameters of the sample due        to the object are displayed with a contrast greater than the        contrast produced in the absence of such substrate.

This invention also relates to the features which will appear clearly inthe following specifications and which will be considered individuallyor according to all their technically possible combinations:

-   -   the sample is irradiated through a wide-aperture lens such as a        microscope lens,    -   the microscope is a microscope with differential interferential        contrast,    -   the microscope is a fluorescence microscope,

This embodiment is most efficient for the display or the detection ofobjects with nanometric dimensions. The purpose is then to displaywithout solving. It enables in particular to visualise all theindividual wire-shaped objects, i.e. distant by a quantity greater thanthe lateral resolution of the microscope, whereof the length is greaterthan a micron (polymers, microtubules, collagen, bacteria, DNA, RNA,carbon nanotubes, nanowires, etc.).

-   -   the thickness e of the stack layer in contact with the object is        such that the complex reflection coefficients r_(p) and r_(s) of        the substrate verify the condition d²/de²[Ln|r_(p)+r_(s)|]=0,    -   the optical properties of the substrate are such that the        minimum value taken by the quantity |r_(p)+r_(s)| over the set        of values of e is as small as possible,    -   the device comprises a polychromatic light source,    -   the device comprises a monochromatic light source,    -   the base is in silicon,

More generally speaking, the base is advantageously an absorbing medium,a metal or a semi-conductor whereof the real part of the optical indexis greater than 3.3,

-   -   the stack consists of a single layer,

This layer is advantageously mineral, composed of a mixture SiO/SiO₂ insuited proportions.

-   -   the layer is a silica layer,    -   the thickness of the silica layer is of the order of 1025 Å, the        incident medium being simply air,    -   the layer is a magnesium fluoride layer,    -   the thickness of the layer of MgF₂ is of the order of 1055 Å,        the incident medium being simply air,    -   the layer is a polymer layer,    -   the layer is a polymer layer, with an optical index        substantially equal to 1.343, the incident medium being simply        air,    -   the layer is a mineral layer, with an optical index        substantially equal to 1.74, the incident medium being simply        water,    -   the layer is a mineral layer, with an optical index        substantially equal to 1.945, the incident medium being simply        oil with an optical index 1.5,    -   the layer is discontinuous and composed of silica blocks and        with an index of 1.343, of the same height delineating the        thickness of the layer and with cross-sectional dimensions        significantly smaller than a micrometer, the incident medium        being simply air,    -   the layer is a mesoporous or nanoporous mineral or organic layer        with an index substantially equal to 1.343, the incident medium        being simply air,    -   the layer is a mineral aerogel with an index substantially equal        to 1.343, the incident medium being simply air,    -   the device comprises a microscope comprising an aperture        diaphragm in the form of a longitudinal slot adjustable around        the axis of the microscope enabling to restrict the illuminating        cone to a single incidence plane in a selected direction,    -   the device comprises a microscope comprising an aperture        diaphragm in the form of a ring limiting the illuminating cone        of the sample around an angle of incidence,    -   the object is a thin film and the stack comprises a bevelled        layer whereof the thickness varies in a monotonic fashion in a        direction X along the surface.

This method and this display device are compatible and may beadvantageously superimposed to any optical scanning microscopy, to anyinvisible light optical technique (UV or IR), to any spectroscopytechnique, to any non-linear optical technique, to any diffusion ordiffraction technique, and to all their combinations. They are inparticular compatible with the fluorescence technique, the micro-Ramantechnique, the confocal microscopy technique, the two-photon microscopytechnique and to all their combinations.

The implementation of the present invention with fluorescence microscopyis particularly advantageous. Indeed, the polarisation of the lightemitted by a fluorescent sample is often different from the polarisationof the incident beam. The fluorescent marker therefore causesdepolarisation of the light at which the device of the invention isparticularly sensitive. Moreover, the extinction factor of the ownincident light of the device of the invention reduces considerably thenoise accompanying the fluorescent signal.

Finally, this implementation of the present invention with fluorescencemicroscopy enables to recognise, among identical fluorescent objects,those which depolarise light, which corresponds to a very peculiarenvironment of the molecules.

This implementation is particularly efficient for the observation ofsurfaces immersed in a fluorescent medium. It is also very advantageousfor reading the fluorescence signal of the biochips, including theobservation of hybridization kinetics.

The invention also concerns a measuring method wherein:

-   -   the display device is cut parallel to the direction X into two        elements,    -   the thin film is deposited on one of these elements,    -   both elements are placed between a cross-reflected polarizer and        analyser under a polarising microscope illuminated by        polychromatic light, in order to form coloured interference        fringes on each of the elements,    -   the offset of the fringes formed respectively in each of the        elements is measured in order to deduce therefrom the properties        of the layer deposited on one of them.

The invention concerns moreover a display device of a sample asdesignated above, wherein the substrate is the bottom of a Petri box.

The invention concerns moreover a device for display of a sample asdesignated above, wherein the sample is a multisensor matrix, whereaseach block or wafer of the matrix may form the last layer of the stack.This multisensor can be a bacteria chip, a virus chip, an antigen chip,an antibody chip, a protein chip, a DNA chip, an RNA chip or achromosome chip, the device constituting then a parallel reading device.

The invention also concerns a method for ellipsometric measurement of asample with spatial resolution under polarising microscope forming apicture of the sample wherein:

-   -   the sample is irradiated by an illuminating beam linearly        polarised via an aperture diaphragm,    -   the light reflected by the sample is analysed by a        polarizer-analyser, characterised by the relative orientation φ        of its polarisation direction relative to that of the polarizer,    -   the intensity reflected is modulated by the relative rotation of        the polarisation of the illuminating beam and of the        polarizer-analyser.

According to this method:

-   -   the aperture diaphragm of the illuminating beam is a ring        centred on the axis of the beam delineating a single angle of        incidence,    -   the average reflected flux φ_(M)(x,y) and its modulation        amplitude φ_(m)(x,y) are measured simultaneously at each point        of the picture obtained of the sample,    -   the measurements φ_(M)(x,y) and φ_(m)(x,y) are processed in        order to deduce therefrom simultaneously at each point of the        sample two combinations of the ellipsometric parameters ψ(x,y)        and Δ(x,y) and of the reflection coefficient |r_(s)|²(x,y) on        the basis of the formulae:

${\frac{1}{2}{r_{s}}^{2}\left( {1 + {\tan^{2}\psi}} \right)} = {{\phi_{M}\mspace{14mu}{and}\mspace{14mu}\frac{1}{2}{r_{s}}^{2}\left( {{\tan^{2}\psi} - {2\;\tan\;{\psi cos}\;\Delta}} \right)} = \phi_{m}}$

-   -   the measurements φ_(M)(x,y) and φ_(m)(x,y) are processed in        order to deduce therefrom the combination sin(2ψ)cos of the        single ellipsometric parameters ψ(x,y) and Δ(x,y) on the basis        of the formula:        φ_(m)=φ_(M)(1−sin(2ψ)cos Δ)

Possibly, in a measuring step:

-   -   the orientation of the analyser relative to the polarizer is set        to a value different from π/2 modulo π,    -   the aperture diaphragm of the illuminating beam is a slot        adjustable around the optical axis of the microscope        superimposed on a ring delineating a single angle of incidence,    -   the intensity of the reflected beam is measured for at least two        different and non-redundant orientations φ of the slot,    -   these measurements of intensity are processed based on the        relation:

$I = {A_{i}^{2}\begin{bmatrix}{{{r_{p}}^{2}\cos^{2}{{\varphi cos}^{2}\left( {\phi - \varphi} \right)}} + {{r_{s}}^{2}\sin^{2}\varphi\;{\sin^{2}\left( {\phi - \varphi} \right)}} +} \\{\left( {{r_{p}r_{s}^{*}} + {r_{p}^{*}r_{s}}} \right)\frac{\sin\; 2\varphi\;{\sin\left( {{2\phi} - {2\varphi}} \right)}}{4}}\end{bmatrix}}$

-   -   the values of both ellipsometric angles ψ(x,y) and Δ(x,y) and        those of the reflection coefficient modules |r_(p)| and |r_(s)|        are deduced therefrom, simultaneously at each point of the        sample.

Possibly, in a complementary step:

-   -   the analyser is fixed in an orientation non-perpendicular to the        polarizer, for example φ=0,    -   the aperture diaphragm of the illuminating beam is a slot        adjustable around the optical axis of the microscope        superimposed on a ring delineating a single angle of incidence,    -   the intensity reflected for both orientations φ=0 and φ=π/2 of        the slot is measured,    -   these measurements of intensity are processed to obtain tan ψ        while taking a square root of their ratio according to the three        formulae:

I = A_(i)²r_(p)²cos²ϕ  for  φ = 0  modulo  π$I = {{A_{i}^{2}{r_{s}}^{2}\sin^{2}\phi\mspace{14mu}{for}\mspace{14mu}\varphi} = {\frac{\pi}{2}\mspace{14mu}{modulo}\mspace{14mu}\pi}}$${\tan\;\psi} = {\frac{r_{p}}{r_{s}}}$

Possibly, in a measuring step:

-   -   the orientation of the analyser relative to the polarizer is set        to a value different from π/2 modulo π,    -   the intensity reflected is modulated by the rotation of the        diaphragm D around the optical axis,    -   the average reflected flux φ_(M)(x,y) and its modulation        amplitude φ_(m)(x,y) are measured simultaneously at each point        of the sample,    -   the measurements φ_(M)(x,y) and φ_(m)(x,y) are processed in        order to deduce therefrom both ellipsometric angles ψ(x,y) and        Δ(x,y) and the modules |r_(p)| and |r_(s)| of the reflection        coefficients based on the relation:

$I = {A_{i}^{2}\begin{bmatrix}{{{r_{p}}^{2}\cos^{2}{{\varphi cos}^{2}\left( {\phi - \varphi} \right)}} + {{r_{s}}^{2}\sin^{2}\varphi\;{\sin^{2}\left( {\phi - \varphi} \right)}} +} \\{\left( {{r_{p}r_{s}^{*}} + {r_{p}^{*}r_{s}}} \right)\frac{\sin\; 2\varphi\;{\sin\left( {{2\phi} - {2\varphi}} \right)}}{4}}\end{bmatrix}}$

Possibly, in a complementary step:

-   -   the orientation of the analyser relative to the polarizer is set        to φ=0,    -   the intensity reflected is modulated by the rotation of the        diaphragm D around the optical axis,    -   the average reflected flux φ_(M)(x,y) and its modulation        amplitude φ_(m)(x,y) are measured simultaneously at each point        of the sample,    -   the measurements φ_(M)(x,y) and φ_(m)(x,y) are processed in        order to deduce therefrom both ellipsometric angles ψ(x,y) and        Δ(x,y) and the modules |r_(p)| and |r_(s)| of the reflection        coefficients based on the relation:

$I = {{A_{i}^{2}\begin{bmatrix}{{{r_{p}}^{2}\cos^{2}{{\varphi cos}^{2}\left( {\phi - \varphi} \right)}} + {{r_{s}}^{2}\sin^{2}\varphi\;{\sin^{2}\left( {\phi - \varphi} \right)}} +} \\{\left( {{r_{p}r_{s}^{*}} + {r_{p}^{*}r_{s}}} \right)\frac{\sin\; 2\varphi\;{\sin\left( {{2\phi} - {2\varphi}} \right)}}{4}}\end{bmatrix}}.}$

The invention also concerns a method for ellipsometric measurement of asample with spatial resolution under polarising microscope forming apicture of the sample, wherein:

-   -   the sample is irradiated by an illuminating beam linearly        polarised via an aperture diaphragm,    -   the light reflected by the sample is analysed by a        polarizer-analyser, characterised by the relative orientation φ        of its polarisation direction relative to that of the polarizer,    -   the intensity reflected is modulated by the relative rotation of        the polarisation of illuminating beam and of the        polarizer-analyser,

According to this method:

-   -   the aperture diaphragm of the illuminating beam is a disk        centred on the axis of this beam,    -   the average reflected flux φ_(M)(x,y) and its modulation        amplitude φ_(m)(x,y) are measured simultaneously at each point        of the picture obtained of the sample,    -   the measurements φ_(M)(x,y) and φ_(m)(x,y) are processed in        order to deduce therefrom simultaneously at each point of the        sample two combinations of the effective ellipsometric        parameters ψ_(eff)(x,y) and Δ_(eff)(x,y) and of the effective        coefficient reflection |r_(s eff)|²(x,y) on the basis of the        formulae:

${\frac{1}{2}{r_{s}}^{2}\left( {1 + {\tan^{2}\psi_{eff}}} \right)} = {{\phi_{M}\mspace{14mu}{et}\mspace{14mu}\frac{1}{2}{r_{s}}^{2}\left( {{\tan^{2}\psi_{eff}} - {2\;\tan\;\psi\;\cos\;\Delta_{eff}}} \right)} = \phi_{m}}$

-   -   the measurements φ_(M)(x,y) and φ_(m)(x,y) are processed in        order to deduce therefrom the combination sin(2ψ)cos of the        single effective ellipsometric parameters ψ_(eff)(x,y) and        Δ_(eff)(x,y) on the basis of the formula:

${\sin\; 2\;\psi_{eff}\cos\;\Delta_{eff}} = {1 - \frac{\phi_{m}}{\phi_{M}}}$

The invention also concerns a device for ellipsometric measurement undermicroscope with lateral spatial resolution.

According to this device:

-   -   it only comprises a single polarizer placed between the        illuminating mirror and the sample on either side of the lens,    -   it comprises a slot revolving in the plane of its aperture        diaphragm, possibly superimposed on a ring diaphragm enabling to        extract the ellipsometric parameters of the sample using at        least three measurements for three different orientations of the        slot and of the formula:

$I = {A_{i}^{2}{r_{s}}^{2}\left\{ {{\tan^{2}\psi\;\cos^{4}\varphi} + {\sin^{4}\varphi} - {\frac{\sin^{2}2\;\varphi}{2}\tan\;\psi\;\cos\;\Delta}} \right\}}$applied to these three measurements, wherein the parameters r_(s), ψ andΔ may be effective parameters derived from averages over all the anglesof incidence present:

$I = {A_{i}^{2}\begin{Bmatrix}{{{r_{p}}^{2}\cos^{2}{{\varphi cos}^{2}\left( {\Phi - \varphi} \right)}} + {{r_{s}}^{2}\sin^{2}\varphi\;{\sin^{2}\left( {\Phi - \varphi} \right)}} +} \\{\left\lbrack {{r_{p}r_{s}^{*}} + {r_{p}^{*}r_{s}}} \right\rbrack\frac{\sin\; 2\varphi\;{\sin\left( {{2\Phi} - {2\varphi}} \right)}}{4}}\end{Bmatrix}}$

According to this device:

-   -   the polarizer and the analyser have a set relative orientation,    -   the aperture diaphragm is a hole or a ring,    -   the picture of the rear focal plane of the lens is formed in the        focal plane object of the ocular by a Bertrand lens,    -   a CCD camera is positioned in this plane,    -   the measurement of the intensity obtained at each point of the        CCD camera is processed thanks to the general formula:

${I\left( {\varphi,\Phi,\theta,\lambda} \right)} = {A_{i}^{2}\begin{Bmatrix}{{{{r_{p}\left( {\theta,\lambda} \right)}}^{2}\cos^{2}{{\varphi cos}^{2}\left( {\Phi - \varphi} \right)}} + {{{r_{s}\left( {\theta,\lambda} \right)}}^{2}\sin^{2}\varphi\;{\sin^{2}\left( {\Phi - \varphi} \right)}} +} \\{\left\lbrack {{{r_{p}\left( {\theta,\lambda} \right)}{r_{s}^{*}\left( {\theta,\lambda} \right)}} + {{r_{p}^{*}\left( {\theta,\lambda} \right)}{r_{s}\left( {\theta,\lambda} \right)}}} \right\rbrack\frac{\sin\; 2\varphi\;{\sin\left( {{2\Phi} - {2\varphi}} \right)}}{4}}\end{Bmatrix}}$in order to obtain directly the set of the ellipsometric parameters ofthe sample.

According to this device:

-   -   a modulation of the intensity reflected is obtained by a        rotation relative of the analyser and of the polarizer,    -   the aperture diaphragm is a hole or a ring,    -   the picture of the rear focal plane of the lens is formed in the        focal plane object of the ocular by a Bertrand lens,    -   a CCD camera or possibly tri-CCD is positioned in this plane,    -   the measurement of the intensity obtained at each point of the        CCD camera or possibly at each point and for each colour        component of the tri-CCD camera is processed thanks to the        general formula:

${I\left( {\varphi,\Phi,\theta,\lambda} \right)} = {A_{i}^{2}\begin{Bmatrix}{{{{r_{p}\left( {\theta,\lambda} \right)}}^{2}\cos^{2}{{\varphi cos}^{2}\left( {\Phi - \varphi} \right)}} + {{{r_{s}\left( {\theta,\lambda} \right)}}^{2}\sin^{2}\varphi\;{\sin^{2}\left( {\Phi - \varphi} \right)}} +} \\{\left\lbrack {{{r_{p}\left( {\theta,\lambda} \right)}{r_{s}^{*}\left( {\theta,\lambda} \right)}} + {{r_{p}^{*}\left( {\theta,\lambda} \right)}{r_{s}\left( {\theta,\lambda} \right)}}} \right\rbrack\frac{\sin\; 2\varphi\;{\sin\left( {{2\Phi} - {2\varphi}} \right)}}{4}}\end{Bmatrix}}$in order to obtain directly the set of the ellipsometric parameters ofthe sample.

According to this device:

-   -   the picture of the rear focal plane of the lens is formed in the        focal plane object of the ocular by a Bertrand lens,    -   a CCD camera is positioned in this plane,    -   the measurement of the intensity obtained at each point of the        CCD camera is processed thanks to the general formula:

${I\left( {\varphi,\Phi,\theta,\lambda} \right)} = {A_{i}^{2}\begin{Bmatrix}{{{{r_{p}\left( {\theta,\lambda} \right)}}^{2}\cos^{2}{{\varphi cos}^{2}\left( {\Phi - \varphi} \right)}} + {{{r_{s}\left( {\theta,\lambda} \right)}}^{2}\sin^{2}\varphi\;{\sin^{2}\left( {\Phi - \varphi} \right)}} +} \\{\left\lbrack {{{r_{p}\left( {\theta,\lambda} \right)}{r_{s}^{*}\left( {\theta,\lambda} \right)}} + {{r_{p}^{*}\left( {\theta,\lambda} \right)}{r_{s}\left( {\theta,\lambda} \right)}}} \right\rbrack\frac{\sin\; 2\varphi\;{\sin\left( {{2\Phi} - {2\varphi}} \right)}}{4}}\end{Bmatrix}}$in order to obtain directly the set of the ellipsometric parameters ofthe sample.

The camera is a tri-CCD colour camera and the measurement of theintensity at each point is performed and processed for each of thecolours.

Advantageously, the object studied is placed on a substrate. Thethickness e of the layer of the stack in contact with the object is suchthat the complex reflection coefficients r_(p) and r_(s) of thesubstrate verify the condition d²/de²[Ln|r_(p)+r_(s)|]=0.

Preferably, the object is placed on a substrate whereof the opticalproperties are such that the minimum value taken by the quantity|r_(p)+r_(s)| over the set of values of e is as small as possible.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will be described more accurately withreference to the appended drawings whereon:

FIGS. 1 and 2 define the polarising parameters of the light p and srelative to the propagation vector k and the orientation parameters,incidence and azimuth θ and φ of the radii in the optical system;

FIG. 3 shows the sample relative to the lens of the microscope;

FIG. 4 is a diagram of the polarising microscope implemented accordingto the invention; wherein FIGS. 4A and 4B are representations of devicesusable according to the invention; the analogous elements arerepresented with the same numeric reference.

FIG. 5 is a schematic diagram of the thickness direct measuring deviceaccording to the invention;

FIGS. 6A and 6B are a diagrammatical representation of the displaydevice of a multisensor implemented in certain embodiments of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

The description of the invention will be made using the notations ofFIGS. 1 and 2, where {right arrow over (p)} is the polarisation vectorof the light carried by a radius of incidence angle θ on the sample.

Besides, by—sample 1—is meant the assembly acting on the measurement.This sample is separate from the lens 2 by an incident medium 3, itcomprises, starting from the incident medium, a studied object 4 (theone that should be displayed), a stack 5 of layers whereof the outermostlayer 6 is the layer in contact with the sample, and a base 7. The stackof layers and the base form the substrate 8.

A sample 1, supposedly plane and isotropic, is therefore placed under areflection-operating optical microscope. The microscope is fitted with alens 10 and a Köhler-type illuminating system 11, comprising at leasttwo lenses 12 and 13 and a aperture diaphragm or pupil 14 conjugated bythe lens 13 of the rear focal plane of the lens 10, represented by adotted line on FIG. 4A. The polarizer P polarises the light directedtoward the sample by the semi-reflecting plate 15. The direction of thepolarizer P is used as a reference. The light sent by the object issubject to an analyser A.

FIG. 4B correspond to the implementation of a differentialinterferential contrast (DIC) microscope, it comprises a polarisingelement 16 which is either a Wollaston biprism or a Nomarksi prism andcompensator.

As it is known besides, it is also possible to replace the linearpolarisations with circular polarisations.

Then, instead of cross-connected polarizers and analysers, there will bethe semi-transparent mirror, a first polarizer, a quarter wave plate(λ/4), the lens, the sample, then as a feedback, the lens again, theplate λ/4, the polarizer mentioned above and the semi-transparentmirror.

In the case of the differential interferential contrast (DIC)microscope, there will be then the semi-transparent mirror, a polarizer,the polarising element, a plate λ/4, the lens, the sample, then as afeedback, the lens again, the plate λ/4, the polarising element, thepolarizer mentioned above and the semi-reflecting mirror.

The angle of incidence of a radius is θ. The microscope is fitted with alinear polarizer and an analyser situated on either side of the sampleon the path of the light. The illumination is episcopic andmonochromatic. The analyser is revolving and forms an angle φ with thepolarizer. The normalised reflected flux Φ_(N) is measured as the ratioof the reflected flux to a reference flux. The reference flux is thatwhich would be obtained on the same instrument adjusted similarly in theabsence of a polarizer and of an analyser with a hypothetical perfectlyreflecting sample. The perfectly reflecting sample is defined by itsFresnel complex coefficients for parallel (p) and perpendicular (s)polarisations as r_(p)=r_(s)=1. For any angle φ:

$\begin{matrix}{{\Phi_{N}\left( {\theta,\phi} \right)} = {{\cos^{2}{\phi\left( {{r_{p}}^{2} + {r_{s}}^{2}} \right)}} - {\frac{\cos\; 2\phi}{4}{{r_{p} + r_{s}}}^{2}}}} & {E1}\end{matrix}$

In the particular case when the polarizer and the analyser arecross-connected (φ=π/2), this formula is reduced to:

$\begin{matrix}{{\Phi_{N}\left( {\theta,\frac{\pi}{2}} \right)} = \frac{{{r_{p} + r_{s}}}^{2}}{4}} & {E2}\end{matrix}$

The second member of the formula (E1) may be construed directly. Itconsists of two terms:

The first, cos² φ(|r_(p)|²+|r_(s)|²), is the product of an extinctioncoefficient and of an intensity reflection coefficient that will now bedesignated as reflectivity. Such reflectivity may be qualified as“incoherent average reflectivity” since it would be obtained by ignoringthe interferences between r_(p) and r_(s), i.e. between the parallel andperpendicular reflected components, and by averaging all the possibleazimuths φ, i.e. over all the possible orientations of the plane ofincidence relative to the direction of the polarizer. Reduced to itsfirst term, the equation (E1) would then provide the reflection obtainedby reversing the order of the sample and of the analyser on the path ofthe light, since the surface only plays here the part of an absorbingelement. This first term disappears totally when φ=π/2: betweencross-connected polarizer and analyser, and in the absence of(de)polarising elements, nothing goes through.

The second term of the equation (E1) describes the interference betweenr_(p) and r_(s). It will be called “coherent reflectivity”. Itrepresents the depolarisation of the incident beam by the surface, whichconverts the linear incident polarisation into an elliptic polarisation.This ellipticity is different for each azimuth, i.e. for each plane ofincidence defined by its angle φ with the direction of the polarizer,and this second term describes the average reflectivity which resultstherefrom for the conical geometry of the irradiation. It disappears forφ=π/4, where the contributions of all the azimuths negate one another,and also for r_(p)=−r_(s). It reduces the total reflectivity betweenparallel polarizer and analyser, and increases them when they arecross-connected.

The display technique, object of the present invention, processesdirectly this second term. We select φ=π/2, and the second term of theequation (EI) remains the only one present. The extinction or, in a moreelaborated version, the quasi extinction of the incoherent reflectivityis one of the foundations of the invention. What is called “coherentreflectivity” may also be called “ellipsometric reflectivity” since itresults from ellipticities (functions of the azimuth φ) of the reflectedpolarisation.

An expression equivalent to (E1) is:

$\begin{matrix}{{\Phi_{N}\left( {\theta,\phi} \right)} = {{\frac{1}{2}\left( {{r_{p}}^{2} + {r_{s}}^{2}} \right)} + {\frac{\cos\; 2\phi}{4}{{r_{p} - r_{s}}}^{2}}}} & {E3}\end{matrix}$

This expression enables to compare the signal obtained in the presenceof polarising elements with the signal obtained in the absence ofpolarising elements, i.e. in non-polarised light, which is provided bythe first term only. It will be noted as:

$\begin{matrix}{{\Phi_{N}\left( {\theta,{NP}} \right)} = {\frac{1}{2}{\left( {{r_{p}}^{2} + {r_{s}}^{2}} \right).}}} & {E4}\end{matrix}$

In the presence of the polarizers, it is still possible to accessexperimentally such quantity by imposing φ=π/4, as shown on the equationE3.

To display the edge of a studied object 4 in the form of a thin filmplaced on the surface, the intensities collected are processed byobserving the film and the bare surface which are noted I_(F) and I_(S).They are proportional to the corresponding normalised fluxes

The contrast of the edge of the film is:

$\begin{matrix}{C = {\frac{I_{F} - I_{S}}{I_{F} + I_{S}} = {1 - \frac{2}{1 + \frac{I_{F}}{I_{S}}}}}} & {E5}\end{matrix}$

To display the film correctly, C should be optimised and therefore theI_(F)/I_(S) ratio should be made maximal (I_(S)→0, to tend toward acontrast of 1) or minimal (I_(F)→0, to tend toward a contrast of −1). Itis then necessary to extinguish either the surface or the film. Thus, asensitive process rests on the one hand on a correct extinction and onthe other hand on a selective extinction.

Our technique combines two extinction factors:

-   -   i) the polarizer and the analyser which are cross-connected or        almost cross-connected,    -   ii) an antireflection substrate for this mode of observation.

The equation (E3) underlines the double nature of our extinction: thecross-connected polarizer and analyser extinguish the first term of thesecond member, our antireflection substrate extinguishes the second. Itcan then be defined as an antireflection substrate for coherentreflectivity. It is the second foundation of our display technique.

But good extinction does not suffice for a sensitive display. I_(F) orI_(S) should be extinguished, but not both at the same time. As the filmdisplayed is very thin, as therefore all its physical parameters hardlydisturb those of the bare surface, it means that the extinction must becritical. In other words, the extinction must be lost for a very smallmodification of the surface. This critical element of the antireflectionquality of our substrate is the third foundation of our displaytechnique.

The performances of a display process may be quantified by the contrastobtained when the film observed becomes extremely thin. In such a case,I_(F) and I_(S) becomes neighbours and dI=I_(F)−I_(S) is close to adifferential element.

C can then be written as:

$C = {{\frac{1}{2}\frac{dI}{I_{S}}} = {\frac{1}{2}\frac{I}{I_{S}}\frac{\mathbb{d}I}{\mathbb{d}e}\Delta\; e}}$where Δe is the thickness of the film which can be assumed when theoptical index is identical to that of the upper layer, and where dI/deis the derivate of the intensity reflected by the bare substraterelative to the thickness e of the last layer of the stack. In the casewhere the substrate is composed of a solid base covered with a singledielectric layer, e is therefore the thickness of this layer. Taking anidentical index for the film and for the last dielectric layer is notcompulsory, but it simplifies the explanation and shows that our processdoes not take advantage of the reflection between the film and thesubstrate. The film is therefore considered here as a simple thicknessfluctuation of the outermost layer.

The sensitivity of our technique is expressed in Angströms⁻¹ as theratio of C to Δe:

$\begin{matrix}{\frac{C}{\Delta\; e} = {{\frac{1}{2}\frac{{\mathbb{d}{Ln}}\; 1}{\mathbb{d}e}} = {{cste} + \frac{{\mathbb{d}{Ln}}{{r_{p} + r_{s}}}}{\mathbb{d}e}}}} & {E7}\end{matrix}$

The expression of r_(p) and r_(s) for a solid covered with a singlelayer is conventional (ref. AZZAM for instance):

$\begin{matrix}{r_{(k)} = \frac{r_{01{(k)}} + {r_{12{(k)}}{\mathbb{e}}^{{- 2}\;{j\beta}\; 1}}}{1 + {r_{01{(k)}} \times r_{12{(k)}}{\mathbb{e}}^{{- 2}\; j\;\beta\; 1}}}} & {E8}\end{matrix}$with k=either s or p, according to the polarisation considered and with

${\beta_{1} = {2\;\pi\;\frac{N_{1}e}{\lambda}\cos\;\theta_{1}}},$the index 1 referring to the layer, the index 2 to the base and theindex 0 to the incident medium.

This equation enables to write:

$\begin{matrix}{{\sigma \equiv {r_{p} + r_{s}}} = \frac{\sigma_{01} + {{\sigma_{12}\left( {1 + \Pi_{01}} \right)}{\mathbb{e}}^{{- 2}\; j\;\beta\; 1}} + {\sigma_{01}\Pi_{12}{\mathbb{e}}^{{- 4}\; j\;\beta\; 1}}}{\left( {1 + {r_{01{(p)}}r_{12{(p)}}{\mathbb{e}}^{{- 2}\; j\;\beta\; 1}}} \right)\left( {1 + {r_{01{(s)}}r_{12{(s)}}{\mathbb{e}}^{{- 2}\; j\;\beta\; 1}}} \right)}} & {E9}\end{matrix}$where σ_(ij) and Π_(ij) represent respectively the sum and the productof r_(ij(p)) and r_(ij(s)).

The sum σ is a periodical function of the optical thickness N₁e ofperiod λ/2. Its module |σ| shows generally two minima and two maxima perperiod. The same goes for Ln|σ|. The function |σ| being moreover adelineated function, it remains very regular and its derivate relativeto e is never very significant. Conversely, the function Ln|σ| divergeswhen |σ| tends toward zero and the sensitivity given by the equation E7becomes very significant in absolute value on either side of the minimumwhen the extinction is total. The contrast is always negative on theleft-hand side of a minimum and positive on the right-hand side.Consequently the condition to obtain a minimum will be designated as a“condition of contrast reversal”.

As a summary, the rapid contrast reversals inversions correspondtherefore to the minima of |r_(p)+r_(s)| relative to e, and the veryrapid contrast reversals are obtained when this minimum of |r_(p)+r_(s)|tends toward 0.

The equation (E3) underlines the significance of the use of afluorescence microscope: in the presence of a fluorescence signal, thedepolarised component of this fluorescence is added to the right-handside member of the equation (E3) without altering the extinction of bothother terms. The signal/noise ratio is therefore increased. This is alsotransposable to a Raman signal.

The invention also concerns an ellipsometric measurement method whichmay also operate without needing to resort to a particular substrate:

The ellipsometric angles ψ and Δ are defined by:

$\begin{matrix}{\frac{r_{p}}{r_{s}} = {\tan\;\Psi\;{\mathbb{e}}^{j\;\Delta}}} & {E10}\end{matrix}$

Two equations selected arbitrarily among the four following ones sufficeto establish the matches which will be useful between reflectivities andellipsometric parameters:|r _(p)|² +|r _(s)|² =|r _(s)|²(1+tan² ψ)  E11|r _(p) +r _(s)|² =|r _(s)|²(1+tan² ψ+2 tan ψ cos Δ)  E12|r _(p) −r _(s)|² =|r _(s)|²(1+tan² ψ−2 tan ψ cos Δ)  E13r _(p) r _(s) *+r _(p) *r _(s)=2|r _(s)|² tan ψ cos Δ  E14

The first of these equations shows that the ellipsometric parameter ψ isaccessible by the measurement of the incoherent reflectivity. Each ofthe other three shows that the determination of the second ellipsometricparameter, Δ, requires moreover to measure the coherent reflectivity (ora combination of both reflectivities). Accessing the coherentreflectivity signal enables to determine ψ and Δ.

The measurement takes place in two steps:

i) The first is based upon the rotation of the analyser. The picture ofthe sample is analysed by a CCD camera or any other two-dimensionaldetector. The equation E3 shows that the signal reflected oscillates ina sine wave fashion around the incoherent reflectivity with an amplitude|r_(p)−r_(s)|² and a period π on the angle φ. Various procedures,requiring at least two measurements, enable to obtain two combinationsof the three parameters |r_(s)|², tan ψ, and cos Δ, for example|r_(s)|²(1+tan² ψ) and 2|r_(s)|² tan ψ cos Δ. This enables already todetermine the combination sin 2ψ cos Δ of the single ellipsometricparameters, but does not suffice to determine Δ and ψ separately.

ii) The second step requires breaking the radial symmetry of theillumination, which may be performed in two ways:

either by modifying physically the geometry of the aperture diaphragm,which must become a slot or a cross composed of two perpendicular slots,or an angular sector δφ (modulo π) with aperture strictly smaller thanπ/4, whereof the apex is confused with the optical axis or theassociation of two or four identical angular sectors, spaced regularlyaround the optical axis of the microscope, capable as the analyser torevolve around the optical axis of the microscope,

or by analysing the distribution of intensity present in a conjugatedplane of the aperture diaphragm placed on the path of the reflectedlight, the microscope being in Koehler illumination. The microscopebeing fitted with a CCD camera in order to receive the picture of thesample, this analysis may take place very simply by interposing aBertrand lens between the lens and the pupil of the camera. It istherefore a conoscopic measurement. The interest of this solution, easyto be implemented, is that the angle of incidence θ and the azimuth φare, in the conjugated plane, two parameters which can be separatedgeographically and that it is then possible to access the whole functionΦ_(N)(θ,φ,λ), λ designating the wave length of the luminous beam. Therange of aperture angles kept may be adjusted, the azimuth explored, orthe illumination filtered by digital means. This solution also enables,in the absence of the Bertrand lens, to carry out the first step of theanalysis simultaneously in several regions of a heterogeneous sample,and hence to determine by parallel measurement the quantity (sin 2ψ cosΔ)(x,y). For complete analysis with the Bertrand lens, it is howevernecessary to select a homogeneous region of the sample by the use of afield diaphragm or of a confocal geometry. This solution does not enabletherefore complete parallel analysis of the different points of thesample. The first solution, conversely (diaphragm with broken radialsymmetry), enables total parallel analysis since the picture of thesample is always kept on the CCD camera.

The intensity reflected I when a very small angular sector δφ selects aparticular azimuth φ on the illuminating cone, provided by theequivalent of the equations EI to E4 is now:

$I = {A_{i}^{2}\begin{bmatrix}{{{r_{p}}^{2}\cos^{2}\varphi\;{\cos^{2}\left( {\phi - \varphi} \right)}} + {{r_{s}}^{2}\sin^{2}\varphi\;{\sin^{2}\left( {\phi - \varphi} \right)}} +} \\{\left( {{r_{p}r_{s}^{*}} + {r_{p}^{*}r_{s}}} \right)\frac{\sin\; 2{{\varphi sin}\left( {{2\phi} - {2\varphi}} \right)}}{4}}\end{bmatrix}}$

Generally speaking, this intensity is a periodical function φ of periodπ and also comprises terms of period π/2.

If the relative orientation of the analyser and of the polarizer is setand:

-   -   if the slot is driven by a uniform rotational movement around        the optical axis at the frequency ω, the intensity reflected by        each point of the sample is modulated and this modulation        enables to extract different combinations of the quantities        |r_(s)|, ψ and Δ requested. To do so, several techniques can be        implemented, notably photometric-type techniques processing        time-related intensity averages and extreme amplitudes or        synchronous detection techniques enabling to compare the        amplitudes and the phases of the components of the intensity        reflected at 2ω and at 4ω;    -   if the orientation of the slot is adjustable manually, the        intensities collected may be measured for several orientations        of the slot, two at least, in order to deduce from the general        formula above, the values of different combinations of the        parameters |r_(s)|, ψ and Δ, which enables to determine        completely the values of these parameters;    -   if the analyser is driven by a uniform rotational movement        around the optical axis, the signal I is modulated with a period        π (over φ) and the measurement of I for different values of φ        becomes more accurate;    -   if finally the analyser and the slot are both driven by uniform        rotation, at different frequencies, the function I(φ, φ) can be        recognised completely and the parameters |r_(s)|, ψ and Δ can be        determined with very great accuracy by a conventional        three-parameter numeric adjustment procedure.

In the simplest particular case when the angle φ is set and when themeasurement of I is performed for both orientations φ=0 (modulo π) andφ=π/2 (modulo π) of the slot, the following can be obtainedrespectively:Φ_(N)(θ, φ, φ=0)=½|r _(p)|² cos² φ and Φ_(N)(θ, φ, φ=π/2)=½|r _(s)|²cos² φ

It suffices therefore to take the root of the ratio of both theseintensities to obtain the quantity tan ψ. This measurement combined toboth previous ones enables therefore to determine completely |r_(s)|²,ψ, and Δ, and hence also |r_(p)|².

It should be noted that the determination of the single parameters ψ andΔ can be obtained by using exclusively ratios with measured intensities,and consequently does not require using any reference substrates.

An interesting particular case is that when Φ=0 which corresponds to aparallel polarizer and analyser and which can therefore be deduced fromthat of a single polarizer arranged between the illuminating mirror andthe lens or even between the lens and the sample. The intensityreflected can be written in such a case as:

$I = {A_{i}^{2}\left\{ {{{r_{p}}^{2}\cos^{4}\varphi} + {{r_{s}}^{2}\sin^{4}\varphi} - {\frac{\sin^{2}2\;\varphi}{2}{r_{s}}^{2}\tan\;{\psi cos}\;\Delta}} \right\}}$

This demonstrates that with a revolving diaphragm composed of a slot, ofa cross consisting of two perpendicular slots crossing one another ontheir axis, of an angular sector whereof the apex is placed on theoptical axis of the microscope and of azimuth amplitude smaller than 45degrees, or of the association of two or four angular sectors of thesame type, arranged regularly around the optical axis, which diaphragmis possibly superimposed to a ring to delineate a single angle ofincidence, it suffices to carry out three measurements of intensityreflected with three different and non-redundant orientations of thediaphragm in order to deduce therefrom the set of the ellipsometricparameters of the sample. For instance, in case when the diaphragm is aslot or a very small angular sector marked by its orientation φ, theintensity reflected at each point of the picture of the sample becomes:

$\begin{matrix}{{I_{1} \equiv {I\left( {\varphi = 0} \right)}} = {{A_{i}^{2}{r_{p}}^{2}} = {A_{i}^{2}{r_{s}}^{2}\left( {1 + {\tan^{2}\psi}} \right)}}} & {{{for}\mspace{14mu}\varphi} = 0} \\{{I_{2} \equiv {I\left( {\varphi = \frac{\pi}{2}} \right)}} = {A_{i}^{2}{r_{s}}^{2}}} & {{{for}\mspace{14mu}\varphi} = {\pi/2}} \\{{I_{3} \equiv {I\left( {\varphi = \frac{\pi}{4}} \right)}} = {2\; A_{i}^{2}{r_{s}}^{2}\tan\;\psi\;\cos\;\Delta}} & {{{for}\mspace{14mu}\varphi} = {\pi/4}}\end{matrix}$

It suffices therefore to calculate the ratio

$\frac{I_{1}}{I_{2}}$in order to deduce therefrom tan ψ, then the ratio

$\frac{I_{3}}{I_{2}}$in order to deduce therefrom cos Δ.

This example illustrates:

-   -   how the measurement of three intensities with three different        orientations of the slot enables to determine the set of the        ellipsometric parameters by using exclusively intensity ratios,        hence without an appended calibration;    -   how a modulation of the intensity reflected which includes these        three measurements, but also others, enables to obtain the same        information with increased accuracy;    -   how an ellipsometer under optical microscope or under binocular        magnifying glass may be realised by using a single polarizer and        a revolving slot.    -   When exposing the measurement process, equations valid for a        single angle of incidence θ have been used. As |r_(s)|², ψ, and        Δ depend on θ, the following options are considered: either        access these quantities for a single angle θ by using an annular        aperture diaphragm, or access quantities averaged over a range        of incidence angles [θ_(min), θ_(max)], with θ_(min)=0, most        often.

The same formulae apply to effective quantities, allocated below with anindex “eff”, defined on the basis of averages over θ. It is thennecessary to state:

r_(p)_(eff)² = ⟨r_(p)²⟩_(θ) r_(s)_(eff)² = ⟨r_(s)²⟩_(θ)${\tan\;\psi_{eff}} = \frac{{r_{p}}_{eff}}{{r_{s}}_{eff}}$${\cos\;\Delta_{eff}} = \frac{\left\langle {{r_{s}}^{2}\;\tan\;{\psi cos}\;\Delta} \right\rangle_{\theta}}{{r_{s}}_{eff} \cdot {r_{p}}_{eff}}$to write the intensity reflected as:

$I = {A_{i}^{2}\begin{bmatrix}{{{r_{p}}_{eff}^{2}\cos^{2}{{\varphi cos}^{2}\left( {\phi - \varphi} \right)}} + {{r_{s}}_{eff}^{2}\sin^{2}\varphi\;{\sin^{2}\left( {\phi - \varphi} \right)}} +} \\{2{r_{s}}_{eff}^{2}\tan\;\psi_{eff}\cos\;\Delta_{eff}\frac{\sin\; 2\varphi\;{\sin\left( {{2\phi} - {2\varphi}} \right)}}{4}}\end{bmatrix}}$

The measurement of I enables therefore to determine the effectivequantities and, in particular, the ellipsometric angles ψ_(eff) andΔ_(eff), which may be compared with values calculated in order to deducetherefrom the properties of the object or of the sample, as it is doneconventionally with the single incidence ellipsometric angles.

Indeed, the interest of the method lies especially in carrying outellipsometric measurements under microscope in order to combineellipsometric measurement and imaging. It should therefore be consideredthat the natural geometry of the illumination is a light cone around thenormal. Still, the ellipsometric parameters hardly vary for smallincidences. This explains why the ellipsometry is a technique onlysensitive at high angles of incidence. The counterpart in our method isthat the average performed on the illuminating cone garbles onlysligthly the signal exploitable. In the absence of optimised substrate,the drawback of the ellipsometric measurement under microscope is thatits sensitivity is poor. But in the presence of the optimised substratesas suggested, the sensitivity of the measurement to the physicalparameters of the sample becomes excellent again, comparable in factwith that of conventional measurements around the Brewster angle,whereas the signal exploited remains little sensitive to the angle ofincidence. This can be explained because between cross-connectedpolarizer and analyser, the extinction of the coherent reflectivity isstill complete in normal incidence, to the extent that only the non-zeroincidences take part to the construction of the signal exploited. Withthe conditions of a good extinction for non-zero incidences, theextinction is good over the set of incidences of the illuminating cone.

It is possible to optimise the thickness of the last layer for anymaterials.

Indeed, it has been shown that the function |σ(e)|=|r_(p)+r_(s)| stillhas more or less marked minima, which correspond to the conditions ofcontrast reversal. The contrast is therefore nil for these particularvalues of e. Being also periodical and continuous, it reaches a minimumon the left-hand side of these values and a maximum on the right-handside. It is therefore still possible to select the thickness e so thatone of these extreme is reached. Regardless of the nature of thesubstrate, the thickness of the dielectric layer can be optimised bycalculating |σ(e)|. This becomes particularly interesting when gettingcloser to the critical conditions.

The critical compositions of the substrates are defined by the existenceof a solution to the equation |σ(e)|=0. A critical substrate possesses alayer thickness close to a solution of this equation. A solution to theequation |σ(e)|=0 corresponds necessarily to a minimum of |σ(e)|. It istherefore a contrast reversal thickness. The smallest of these values,e_(c), plays of course a particular part. The other contrast reversalthicknesses are then given by e_(c),k=e_(c)+KN₁λ/2.

According to the equation E9, the equation discussed is:σ₀₁+σ₁₂(1+Π₀₁)e ^(−2jβ1)+σ₀₁Π₁₂ e ^(−4jβ1)=0  E15

In the case of a stack reduced to a single layer, the values e_(c) of eare the solutions of the equation E15 derived from of E9:σ₀₁+σ₁₂(1+Π₀₁)z+σ₀₁Π₁₂ z ²=0  E16

It possesses two complex solutions z1 and z2 which are functions of theindices of the incident medium, of the layer, of the complex index ofthe substrate, and of the angle of incidence θ0 (or in an equivalentfashion, of the angle refracted in the layer, θ1). The criticalconditions are reached when the module of one of both these solutions isequal to 1. This problem is rather simple to be explored numerically.Analytically, it is possible to develop each of the terms up to theorder 4 in θ1 since each of them depends only little on the angle closeto the normal. “Taylor-made” solutions can then be found. In practice,both extreme media are often imposed and it is the index and thethickness of the layer that should be determined. The contrast is thenplotted in relation to the thickness for a few random indices and amonotonic variation in the contrast can be observed. It sufficestherefore to progress in the direction when things improve until theybegin to fall off in quality. From then on, the exploration is resumedaround the best value while fine-tuning the index variations. Thenumerous results published in the literature may also be used for asingle layer in terms of ψ and Δ. The situations requested correspondsimultaneously to:tan ψ=1 and Δ=π  E17

The solutions found numerically are rather well approximated by theempiric formula:

$\begin{matrix}{N_{2} = {N_{o}\left\lbrack {\frac{N_{0}}{N_{3}} + \sqrt{1 + \left( \frac{N_{0}}{N_{3}} \right)^{2}}} \right\rbrack}} & {E18}\end{matrix}$

Particularly interesting results are obtained by realising a siliconsubstrate covered with a single layer meeting the following parameters,the irradiation being preferably monochromatic and of wavelength λ=540nm and the angle of aperture of the illumination cone being assumed as30 degrees:

N_(o) N₃ N₂ e 1.00 (air) 3.88 − 0.02 i 1.343 1060 ± 5 Å 1.34 (water)3.88 − 0.02 i 1.749  814 ± 5 Å 1.50 (oil) 3.88 − 0.02 i 1.945  750 ± 10Åwhen N₀ is the index of the ambient medium, N₃ is the index of the baseof the substrate, N₂ is the index of the layer and e its thickness.

The optimal thickness e is a linear function of λ but cannot be madeproportional to λ. For observations in the air,

$\frac{\partial e}{\partial\lambda} = {0.2.}$

The layers with indices 1.74 and 1.945 may be produced by numerousmethods, such as the PECVD deposits. The layers with an index of 1.345are more difficult to realise. They may be formed of a hydrogel, of anaerogel, of a polymer or be heterogeneous, for example formed of blockswith constant thickness and with very small dimensions. It may also be asolution in water, sugar, salt, polymer. . .

A particularly interesting method for visualising the optical thickness(N₁×e₁) of a very thin film can be realised with the substrateimplemented in the invention as follows, as represented on FIG. 5.

On a base 20 is realised a deposit of a layer 21 of variable thicknessin the form of a chamfer (FIG. 5A, FIG. 5B).

This substrate 20 is then cut in order to obtain two identical elements22, 23 (23 non-represented is then identical to 22) (FIG. 5C).

One of these elements of the thin film 24 to be studied (FIG. 5C) isthen covered.

Both these elements under microscope, irradiated with white light usinga disk-shaped pupil, are then observed, after having positioned boththese elements with respect to one another, in their relative initialposition, using a mark, a notch or a wedge 25.

White light fringes 26, 27 are then observed, respectively on each ofthese elements and their relative offset Δl enables to measure theproperties of the layer deposited on one of these elements.

The invention is particularly suited to the display of elementscontained in multisensors.

A chemical or biological (biochip) multisensor consists of a base 30whereon are deposited wafers 31 (=spots) each formed of a differentlayer capable of fixing selectively a different specie to be recognisedwithin a liquid mixture (biochip) or gaseous (artificial nose), andforming between them a matrix of elements arranged along the surface.Each wafer has a surface area of a few square microns and often athickness of molecular order of magnitude.

The multisensor is used as follows: it is put in contact with themixture that should be analysed. Each wafer 31, 32 . . . captures thespecie which it can recognise when it is present in the mixture. In situor after rinsing, the purpose is to discover which blocks 32 have becomeloaded and which blocks 31 have remained empty to know the compositionof the mixture. A fixed specie creates an excessive thickness, which canbe visualised at the block, the position of the blocks 31, 32 in thematrix tells about the nature of the specie recognised. This step is thereading step of the multisensor.

Our microscopy method is sensitive enough to show the difference betweenan empty block and the same block when loaded, in many types ofmultisensors. It provided therefore a simple reading method, direct andparallel for all the multisensors.

In a preferred example, its implementation is described on a particulartype of multisensors: the biochips. They comprise for example DNA chips,antibody chips, bacteria chips, virus chips, chromosome chips, proteinchips, etc.

In the example of DNA chips, each block consists of a molecular layerwith identical oligonucleotides capable hybridization with and only withtheir complementary strand. The DNA analysed is cut into strand ofsuitable length, amplified by the PCR technique, which means that eachstrand is replicated a great number of times, then turned into asolution in contact with the chip. The strands recognised are fixed bythe corresponding wafers.

Our method enables to recognise the loaded wafers. In this view, thewafers whereof the thickness is regular and known, are taken as elementsof the multilayer edifice, so that the assembly composed ofbase+multilayer+wafer or block constitutes a very high sensitivityoptimised substrate. Under these conditions, the presence of additionalstrands after hybridization is detected easily by intensity or colourloading implied in the observation of the wafer by our display method.The quantity of material present on a wafer may also be assessedquantitatively by our measurement method.

1. A method for ellipsometric measurement of a sample with spatialresolution under polarising microscope forming a picture of the sample,wherein: the sample is irradiated by an illuminating beam linearlypolarised via an aperture diaphragm, the light reflected by the sampleis analysed by a polarizer/analyser, characterised by the relativeorientation φ of its polarisation direction relative to that of thepolarizer, the intensity reflected is modulated by the relative rotationof the polarisation of the illuminating beam and of thepolarizer-analyser, characterised in that: the aperture diaphragm of theilluminating beam is a ring centred on the axis of the beam delineatinga single angle of incidence, the average reflected flux φ_(M)(x,y) andits modulation amplitude φ_(m)(x,y) are measured simultaneously at eachpoint of the picture obtained of the sample, the measurements φ_(M)(x,y)and φ_(m)(x,y) are processed in order to deduce therefrom simultaneouslyat each point of the sample two combinations of the ellipsometricparameters ψ(x,y) and Δ(x,y) and of the reflection coefficient|r_(s)|²(x,y) on the basis of the formulae:${\frac{1}{2}{r_{s}}^{2}\left( {1 + {\tan^{2}\psi}} \right)} = {{\phi_{M}\mspace{14mu}{and}\mspace{14mu}\frac{1}{2}{r_{s}}^{2}\left( {{\tan^{2}\psi} - {2\;\tan\;{\psi cos}\;\Delta}} \right)} = \phi_{m}}$the measurements φ_(M)(x,y) and φ_(m)(x,y) are processed in order todeduce therefrom the combination sin(2ψ) cos of the single ellipsometricparameters ψ(x,y) and Δ(x,y) on the basis of the formula:φ_(m)=φ_(M)(1−sin(²ψ) cos Δ).
 2. A method for ellipsometric measurementof a sample with space resolution under polarising microscope accordingto claim 1, the object studied being placed on a substrate comprising anobject, characterised in that the thickness e of the layer of the stackin contact with the object is such that the complex reflectioncoefficients r_(p) and r_(s) of the substrate verify the conditiond²/de²[Ln|r_(p)+r_(s)|]=0.
 3. A method for ellipsometric measurement ofa sample with space resolution under polarising microscope according toclaim 1, characterised in that the object is placed on a substratewhereof the optical properties are such that the minimum value taken bythe quantity |r_(p)+r_(s)| over the set of values of e is as small aspossible.
 4. A method for ellipsometric measurement of a sample withspace resolution under polarising microscope forming a picture of thesample, wherein: the sample is illuminated by an illuminating beambiased in a linear fashion via an aperture diaphragm, the lightreflected by the sample is analysed by a polarizer/analyser,characterised by the relative orientation φ of its polarisationdirection relative to that of the polarizer, the intensity reflected ismodulated by the relative rotation of the polarisation of illuminatingbeam and of the polarizer-analyser, characterised in that, in ameasuring step: the orientation of the analyser relative to thepolarizer is set to a value different from π/2 modulo π, the aperturediaphragm of the illuminating beam is a slot adjustable around theoptical axis of the microscope, superimposed on a ring delineating asingle angle of incidence, the intensity of the reflected beam ismeasured for at least two different and non-redundant orientations φ ofthe slot, said measurements of intensity are processed on the basis ofthe relation: $I = {A_{i}^{2}\begin{bmatrix}{{{r_{p}}^{2}\cos^{2}{{\varphi cos}^{2}\left( {\phi - \varphi} \right)}} + {{r_{s}}^{2}\sin^{2}\varphi\;{\sin^{2}\left( {\phi - \varphi} \right)}} +} \\{\left( {{r_{p}r_{s}^{*}} + {r_{p}^{*}r_{s}}} \right)\frac{\sin\; 2\varphi\;{\sin\left( {{2\phi} - {2\varphi}} \right)}}{4}}\end{bmatrix}}$ the values of both ellipsometric angles ψ(x,y) andΔ(x,y) and those of the reflection coefficient modules |r_(p)| and|r_(s)| are deduced therefrom, simultaneously at each point of thesample.
 5. A method for ellipsometric measurement of a sample with spaceresolution under polarising microscope according to claim 4,characterised in that, in a complementary step: the analyser is fixed inan orientation non-perpendicular to the polarizer, for example φ=0, theaperture diaphragm of the illuminating beam is a slot adjustable aroundthe optical axis of the microscope superimposed on a ring delineating asingle angle of incidence, the intensity reflected is measured for bothorientations φ=0 and φ=π/2 of the slot, said measurements of intensityare processed to obtain tan ψ while taking a square root of their ratioaccording to the three formulae:I = A_(i)²r_(p)²cos²ϕ   for  φ = 0  modulo  π$I = {{A_{i}^{2}{r_{s}}^{2}\sin^{2}\phi\mspace{40mu}{for}\mspace{14mu}\varphi} = {\frac{\pi}{2}\mspace{14mu}{modulo}\mspace{14mu}\pi}}$${\tan\;\psi} = {{\frac{r_{p}}{r_{s}}}.}$
 6. A method forellipsometric measurement of a sample with space resolution underpolarising microscope according to claim 5, characterised in that, in acomplementary step: the orientation of the analyser relative to thepolarizer is set to φ=0, the intensity reflected is modulated by therotation of the diaphragm D around the optical axis, the averagereflected flux φ_(M)(x,y) and its modulation amplitude φ_(m)(x,y) aremeasured simultaneously at each point of the sample, the measurementsφ_(M)(x,y) and φ_(m)(x,y) are processed in order to deduce therefromboth ellipsometric angles ψ(x,y) and Δ(x,y) and the modules |r_(p)| and|r_(s)| of the reflection coefficients based on the relation:$I = {{A_{i}^{2}\begin{bmatrix}{{{r_{p}}^{2}\cos^{2}{{\varphi cos}^{2}\left( {\phi - \varphi} \right)}} + {{r_{s}}^{2}\sin^{2}\varphi\;{\sin^{2}\left( {\phi - \varphi} \right)}} +} \\{\left( {{r_{p}r_{s}^{*}} + {r_{p}^{*}r_{s}}} \right)\frac{\sin\; 2\varphi\;{\sin\left( {{2\phi} - {2\varphi}} \right)}}{4}}\end{bmatrix}}.}$
 7. A method for ellipsometric measurement of a samplewith space resolution under polarising microscope according to claim 4,the object studied being placed on a substrate comprising an object,characterised in that the thickness e of the layer of the stack incontact with the object is such that the complex reflection coefficientsr_(p) and r_(s) of the substrate verify the conditiond²/de²[Ln|r_(p)+r_(s)|]=0.
 8. A method for ellipsometric measurement ofa sample with space resolution under polarising microscope according toclaim 4, characterised in that the object is placed on a substratewhereof the optical properties are such that the minimum value taken bythe quantity |r_(p)+r_(s)| over the set of values of e is as small aspossible.
 9. A method for ellipsometric measurement of a sample withspace resolution under polarising microscope forming a picture of thesample, wherein: the sample is illuminated by an illuminating beambiased in a linear fashion via an aperture diaphragm, the lightreflected by the sample is analysed by a polarizer/analyser,characterised by the relative orientation φ of its polarisationdirection relative to that of the polarizer, the intensity reflected ismodulated by the relative rotation of the polarisation of illuminatingbeam and of the polarizer-analyser, characterised in that, in ameasuring step: the orientation of the analyser relative to thepolarizer is set to a value different from π/2 modulo π, the intensityreflected is modulated by the rotation of the diaphragm D around theoptical axis, the average reflected flux φ_(M)(x,y) and its modulationamplitude φ_(m)(x,y) are measured simultaneously at each point of thesample, the measurements φ_(M)(x,y) and φ_(m)(x,y) are processed inorder to deduce therefrom both ellipsometric angles ψ(x,y) and Δ(x,y)and the modules |r_(p)| and |r_(s)| of the reflection coefficients basedon the relation: $I = {{A_{i}^{2}\begin{bmatrix}{{{r_{p}}^{2}\cos^{2}{{\varphi cos}^{2}\left( {\phi - \varphi} \right)}} + {{r_{s}}^{2}\sin^{2}\varphi\;{\sin^{2}\left( {\phi - \varphi} \right)}} +} \\{\left( {{r_{p}r_{s}^{*}} + {r_{p}^{*}r_{s}}} \right)\frac{\sin\; 2\varphi\;{\sin\left( {{2\phi} - {2\varphi}} \right)}}{4}}\end{bmatrix}}.}$
 10. A method for ellipsometric measurement of a samplewith space resolution under polarising microscope according to claim 9,the object studied being placed on a substrate comprising an object,characterised in that the thickness e of the layer of the stack incontact with the object is such that the complex reflection coefficientsr_(p) and r_(s) of the substrate verify the conditiond²/de²[Ln|r_(p)+r_(s)|]=0.
 11. A method for ellipsometric measurement ofa sample with space resolution under polarising microscope according toclaim 9, characterised in that the object is placed on a substratewhereof the optical properties are such that the minimum value taken bythe quantity |r_(p)+r_(s)| over the set of values of e is as small aspossible.
 12. A method for ellipsometric measurement of a sample withspatial resolution under polarising microscope forming a picture of thesample, wherein: the sample is irradiated by an illuminating beamlinearly polarised via an aperture diaphragm, the light reflected by thesample is analysed by a polarizer/analyser, characterised by therelative orientation φ of its polarisation direction relative to that ofthe polarizer, the intensity reflected is modulated by the relativerotation of the polarisation of illuminating beam and of thepolarizer-analyser, characterised in that: the aperture diaphragm of theilluminating beam is a disk centred on the axis of this beam, theaverage reflected flux φ_(M)(x,y) and its modulation amplitudeφ_(m)(x,y) are measured, simultaneously at each point of the pictureobtained of the sample, the measurements φ_(M)(x,y) and φ_(m)(x,y) areprocessed in order to deduce therefrom, simultaneously at each point ofthe sample, two combinations of the effective ellipsometric parametersψ_(eff)(x,y) and Δ_(eff)(x,y) and of the effective coefficientreflection |r_(s eff)|²(x,y) on the basis of the formulae:${\frac{1}{2}{r_{s}}^{2}\left( {1 + {\tan^{2}\psi_{eff}}} \right)} = {\phi_{M}\mspace{14mu}{and}}$${\frac{1}{2}{r_{s}}^{2}\left( {{\tan^{2}\psi_{eff}} - {2\;\tan\;{\psi cos}\;\Delta_{eff}}} \right)} = \phi_{m}$the measurements φ_(M)(x,y) and φ_(m)(x,y) are processed in order todeduce therefrom the combination sin(2ψ)cos of the single effectiveellipsometric parameters ψ_(eff)(x,y) and Δ_(eff)(x,y) on the basis ofthe formula:${\sin\; 2\psi_{eff}\cos\;\Delta_{eff}} = {1 - {\frac{\phi_{m}}{\phi_{M}}.}}$13. A method for ellipsometric measurement of a sample with spaceresolution under polarising microscope according to claim 12, the objectstudied being placed on a substrate comprising an object, characterisedin that the thickness e of the layer of the stack in contact with theobject is such that the complex reflection coefficients r_(p) and r_(s)of the substrate verify the condition d²/de²[Ln|r_(p)+r_(s)|]=0.
 14. Amethod for ellipsometric measurement of a sample with space resolutionunder polarising microscope according to claim 12, characterised in thatthe object is placed on a substrate whereof the optical properties aresuch that the minimum value taken by the quantity |r_(p)+r_(s)| over theset of values of e is as small as possible.
 15. A device forellipsometric measurement under microscope with lateral spatialresolution, characterised in that: it only comprises a single polarizerplaced between the illuminating mirror and the sample on either side ofthe lens, it comprises a slot revolving in the plane of its aperturediaphragm, possibly superimposed on a ring diaphragm enabling to extractthe ellipsometric parameters of the sample using at least threemeasurements for three different orientations of the slot and of theformula:$I = {A_{i}^{2}{r_{s}}^{2}\left\{ {{\tan^{2}\psi\;\cos^{4}\varphi} + {\sin^{4}\varphi} - {\frac{\sin^{2}2\;\varphi}{2}\tan\;\psi\;\cos\;\Delta}} \right\}}$ applied to these three measurements, wherein the parameters r_(s), ψand Δ may be effective parameters derived from averages on all theangles of incidence present: r_(p)_(eff)² = ⟨r_(p)²⟩_(θ)r_(s)_(eff)² = ⟨r_(s)²⟩_(θ)${\tan\;\psi_{eff}} = \frac{{r_{p}}_{eff}}{{r_{s}}_{eff}}$${\cos\;\Delta_{eff}} = {\frac{\left\langle {{r_{s}}^{2}\tan\;\psi\;\cos\;\Delta} \right\rangle_{\theta}}{{r_{s}}_{eff} \cdot {r_{p}}_{eff}}.}$16. A device for ellipsometric measurement under microscope according toclaim 15, characterised in that: the picture of the rear focal plane ofthe lens is formed in the focal plane object of the ocular by a Bertrandlens, a CCD camera is positioned in this plane, the measurement of theintensity obtained at each point of the CCD camera is processed thanksto the general formula:${I\left( {\varphi,\Phi,\theta,\lambda} \right)} = {A_{i}^{2}\begin{Bmatrix}{{{{r_{p}\left( {\theta,\lambda} \right)}}^{2}\cos^{2}{{\varphi cos}^{2}\left( {\Phi - \varphi} \right)}} + {{{r_{s}\left( {\theta,\lambda} \right)}}^{2}\sin^{2}\varphi\;{\sin^{2}\left( {\Phi - \varphi} \right)}} +} \\{\left\lbrack {{{r_{p}\left( {\theta,\lambda} \right)}{r_{s}^{*}\left( {\theta,\lambda} \right)}} + {{r_{p}^{*}\left( {\theta,\lambda} \right)}{r_{s}\left( {\theta,\lambda} \right)}}} \right\rbrack\frac{\sin\; 2\varphi\;{\sin\left( {{2\Phi} - {2\varphi}} \right)}}{4}}\end{Bmatrix}}$ in order to obtain directly the set of the ellipsometricparameters of the sample.
 17. A device according to claim 16,characterised in that the camera is a tri-CCD colour camera and that themeasurement of the intensity at each point is performed and processedfor each of the colours.
 18. A device for ellipsometric measurementunder microscope, characterised in that: the polarizer and the analyserhave a set relative orientation, the aperture diaphragm is a hole or aring, the picture of the rear focal plane of the lens is formed in thefocal plane object of the ocular by a Bertrand lens, a CCD camera ispositioned in this plane, the measurement of the intensity obtained ateach point of the CCD camera is processed thanks to the general formula:${I\left( {\varphi,\Phi,\theta,\lambda} \right)} = {A_{i}^{2}\begin{Bmatrix}{{{{r_{p}\left( {\theta,\lambda} \right)}}^{2}\cos^{2}{{\varphi cos}^{2}\left( {\Phi - \varphi} \right)}} + {{{r_{s}\left( {\theta,\lambda} \right)}}^{2}\sin^{2}\varphi\;{\sin^{2}\left( {\Phi - \varphi} \right)}} +} \\{\left\lbrack {{{r_{p}\left( {\theta,\lambda} \right)}{r_{s}^{*}\left( {\theta,\lambda} \right)}} + {{r_{p}^{*}\left( {\theta,\lambda} \right)}{r_{s}\left( {\theta,\lambda} \right)}}} \right\rbrack\frac{\sin\; 2\varphi\;{\sin\left( {{2\Phi} - {2\varphi}} \right)}}{4}}\end{Bmatrix}}$ in order to obtain directly the set of the ellipsometricparameters of the sample.
 19. A device for ellipsometric measurementunder microscope according to claim 18, characterised in that: amodulation of the intensity reflected is obtained by relative rotationof the analyser and of the polarizer, the aperture diaphragm is a holeor a ring, the picture of the rear focal plane of the lens is formed inthe focal plane object of the ocular by a Bertrand lens, a CCD camera orpossibly tri-CCD is positioned in this plane, the measurement of theintensity obtained at each point of the CCD camera or possibly at eachpoint and for each colour component of the tri-CCD camera is processedthanks to the general formula:${I\left( {\varphi,\Phi,\theta,\lambda} \right)} = {A_{i}^{2}\begin{Bmatrix}{{{{r_{p}\left( {\theta,\lambda} \right)}}^{2}\cos^{2}{{\varphi cos}^{2}\left( {\Phi - \varphi} \right)}} + {{{r_{s}\left( {\theta,\lambda} \right)}}^{2}\sin^{2}\varphi\;{\sin^{2}\left( {\Phi - \varphi} \right)}} +} \\{\left\lbrack {{{r_{p}\left( {\theta,\lambda} \right)}{r_{s}^{*}\left( {\theta,\lambda} \right)}} + {{r_{p}^{*}\left( {\theta,\lambda} \right)}{r_{s}\left( {\theta,\lambda} \right)}}} \right\rbrack\frac{\sin\; 2\varphi\;{\sin\left( {{2\Phi} - {2\varphi}} \right)}}{4}}\end{Bmatrix}}$ in order to obtain directly the set of the ellipsometricparameters of the sample.
 20. A device according to claim 19,characterised in that the camera is a tri-CCD colour camera and that themeasurement of the intensity at each point is performed and processedfor each of the colours.
 21. A device according to claim 18,characterised in that the camera is a tri-CCD colour camera and that themeasurement of the intensity at each point is performed and processedfor each of the colours.